The invention relates to apparatus for generating and radiating ultrasonic energy in fluids, in particular for irradiating cleaning baths, having an ultrasonic generator and a converter or transducer associated therewith.
Ultrasonic cleaning installations usually include at least three components parts, namely a tank for containing the cleaning fluid or the coupling medium, an electrical ultrasonic generator and an electro-acoustic sound transducer. The latter converts the electrical oscillations produced by the generator, at the same frequency. Such transducers are secured individually or in groups to tank bottoms or walls. In the case of larger-size installations, the transducers are also built into water-tight casings which are then in turn lowered into the bath fluid.
A distinction is made nowadays in particular between two types of transducers, namely on the one hand the magnetostrictive transducers and on the other piezoelectric tranducers.
The magnetorestrictive nickel-iron transducer is historically the earlier type. In the interests of achieving a high level of conversion efficiency, all power transducers are operated at or in the vicinity of their mechanical natural resonance. However, this requirement alone establishes the most important parameters of a transducer and the dimensions thereof, if its frequency is known.
The main disadvantage from which all transducers suffer is the very poor degree of acoustic matching to the load, that is to say, to the bath fluid. In order to improve the matching effect, the radiating surface must be as large as possible. However, that requirement is clearly subjected to limits insofar as the maximum lateral dimensions of the transducer are restricted by the working frequency on the one hand and the speed of propagation in the material on the other hand. A typical piezoelectric transducer for a working frequency of 20 kHz is about 100 mm in length with a lateral dimension of about 65 mm. The characteristic acoustic impedance Zo which primarily determines the matching effect is between 1 and 1.5 kg/m.sup.2 s for fluids (Zo water=1.485 kg/m.sup.2 s), while that of aluminium and steel, which are the materials most widely used, is 17 and 46kg/cm.sup.2 s respectively. Accordingly, there is mismatching of about 1:17 to 1:46. Accordingly, the entire transducer under load has a mechanical oscillation quality of about 17 in the case of aluminium and about 46 in the case of steel. These relatively high levels of oscillation quality under load however mean that only about 2 to 6% of the energy obtainable at the transducer is also actually irradiated. The remainder, from 94 to 98%, circulates as idle power in the transducer and there only causes an undesirable increase in temperature and a poor power factor.
As the transducer is limited mechanically (rupture strength) by the current and electrically by the voltage (dielectric strength), in the interests of making good use of its potential and the investment costs, the transducer must therefore be matched to the best possible extent to the load (cleaning fluid). However, there is still another factor which acts against good matching. The power irradiated per unit of surface area cannot be increased as desired, as cavitation causes partial decoupling which is equivalent to a drop in the Zo of the fluid. The state of the art at the present time now provides that a number of transducers are mounted on the bottoms of tanks or on the immersion oscillator housing and in this way achieves a larger radiation area, with very high investment costs. It will be appreciated that dividing the sound source into separate individual oscillators which are fed by a generator results in an improvement in acoustic homogeneity. It would be very undesirable if the entire ultrasonic power in a bath of about 2 l upwards had to be irradiated from the single acoustic transducer as that would result in a very high degree of non-homogeneity with respect to ultrasonic intensity. It is clear that problems occur here not only because of the poor acoustic matching of the individual transducer to the medium but also in regard to acoustic homogeneity, cavitation, corrosion, service life, reliability, costs or the like.